JP2015009431A - Image formation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image formation device of a low cost which has a stable graduation expression.SOLUTION: An exposure part 30 comprises a lens group 310 having plural lenses which are arranged in a first direction, and a device array 301 which is so disposed as to face the lens group 310 and has plural organic EL elements 302 which are arranged on a substrate 305 parallel to the first direction. A drive circuit 303, which has plural TFT circuits 306 for controlling a brightness of the organic EL elements 302 per a unit time, is disposed on the substrate 305, and the TFT circuit 306 controls the brightness of the organic EL elements 302 on the basis of a signal created by an area graduation method.

Description

  The present invention relates to an image forming apparatus.

  As an exposure head for selectively exposing a photoconductor used in an image forming apparatus such as a printer using an electrophotographic process, a light emitting element array and a microlens array are provided as in Patent Document 1. A configuration is proposed. As the light emitting element, a Light Emitting Diode (LED) element, an organic Electro Luminescence (EL) element, or the like is used. In particular, when the organic EL element array is used for the exposure head, it is not necessary to arrange the light emitting elements with high precision unlike the LED element array, and the cost can be reduced because it can be formed monolithically on the substrate.

  On the other hand, the image signal of each pixel of the light emitting element array is determined by an area gradation method such as a dither method or an error diffusion method in gradation expression of a halftone image. Japanese Patent Application Laid-Open No. 2004-228561 proposes creating an image signal by a multi-value area gradation method.

  As a typical method of gradation control of each light emitting element, there is pulse width modulation for controlling the exposure time.

JP 2011-110762 A Japanese Patent Laid-Open No. 2002-16802

  In order to perform pulse width modulation, the number of thin film transistors (TFTs), which are one of the elements of the peripheral circuit or pixel circuit, is improved, and the substrate area is accompanied by a decrease in yield and an increase in the total area of the TFT manufactured. There is a problem that the increase in cost occurs and the cost increases.

  Accordingly, an object of the present invention is to provide an image forming apparatus that is low in cost and has stable gradation expression.

  The image forming apparatus of the present invention develops the electrostatic latent image as a toner image, a charging unit that charges the photosensitive member, an exposure unit that forms an electrostatic latent image on the surface of the photosensitive member, and the electrostatic latent image. An image forming apparatus comprising: a developing unit; a transfer unit that transfers the toner image onto a transfer material; and a fixing unit that fixes the transferred toner image onto the transfer material. A lens group having a plurality of lenses arranged in a first direction, and an element having a plurality of organic EL elements arranged opposite to the lens group and arranged in parallel with the first direction on a substrate And a drive circuit having a plurality of transistor circuits for controlling the luminance per unit time of the plurality of organic EL elements is arranged on the substrate of the element array, and is created by an area gradation method Based on the received signal. Register circuit and controls the luminance of the organic EL element.

  According to the present invention, it is possible to provide an image forming apparatus that is low in cost and has stable gradation expression.

Schematic showing an example of an image forming apparatus according to the present embodiment Schematic of an exposure head used in the image forming apparatus according to the present embodiment Schematic showing an example of an element array of an exposure head according to the present embodiment Schematic diagram of a circuit for driving an organic EL element when amplitude modulation is performed Schematic diagram of a circuit for driving an organic EL element when performing pulse width modulation Schematic for explaining image processing of the image forming apparatus according to the present embodiment The figure explaining the area gradation method in the case of performing pulse width modulation Diagram explaining gradation change when pulse width modulation is performed The figure explaining the area gradation method in the case of amplitude modulation The figure explaining the gradation change in the case of amplitude modulation Cross-sectional view of exposure distribution by gradation using pulse width modulation Cross-sectional view of exposure distribution by gradation by luminance modulation Relationship diagram between gradation and line width variation in pulse width modulation and luminance modulation FIG. 5 is a view showing an example of a lens group of the exposure head of the present embodiment. The figure explaining the image formation system of the lens group of this embodiment Main array sectional view and sub-array sectional view of the lens group according to this embodiment Main array sectional view showing imaging light flux from each light emitting point position of lens group of comparative example Main array sectional view showing imaging light flux from each light emitting point position of the lens group according to the present embodiment Relationship diagram between light emitting point position and imaging light quantity ratio when using lens group according to comparative example Relationship diagram between light emitting point position and imaging light quantity ratio when the lens group of this embodiment is used

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[Electrophotographic image forming apparatus]
The image forming apparatus of this embodiment will be described with reference to FIG. FIG. 1 is a schematic cross-sectional view of an image forming apparatus 1 of the present embodiment. The image forming apparatus 1 of this embodiment is a full-color laser printer that employs an inline method and an intermediate transfer method.

  The image forming apparatus 1 can form a full-color image on a recording paper 100 (for example, a recording paper, a plastic sheet, a cloth, etc.) according to the image information. The image information is input to the image forming apparatus 1 from an image reading apparatus (not shown) connected to the image forming apparatus 1 or a host device such as a personal computer connected to the image forming apparatus 1 so as to be communicable. The image forming apparatus 1 includes first, second, third, and fourth image forming units SY for forming yellow (Y), magenta (M), cyan (C), and black (K) images. , SM, SC, SK. In the present embodiment, the first to fourth image forming units SY, SM, SC, and SK are arranged in a line in the horizontal direction. In the present embodiment, the configurations and operations of the first to fourth image forming units SY, SM, SC, and SK are substantially the same except that the colors of images to be formed are different. In the following, unless there is a particular distinction, the subscripts Y, M, C, and K given to the reference numerals to represent the elements provided for any color in FIG. 1 are omitted, A general description.

  In the present embodiment, the image forming apparatus 1 includes four drum-type electrophotographic photosensitive members, that is, the photosensitive drums 10 arranged in parallel in the horizontal direction as a plurality of image carriers. The photosensitive drum 10 is rotationally driven by a drive unit (drive source) (not shown) in an arrow direction (clockwise direction) shown in the drawing.

  First, a charging roller 40 as a charging unit is disposed around the photosensitive drum 10. By this charging roller 40, the surface of the photosensitive drum 10 is uniformly charged to a negative polarity.

  Next, an exposure head 30 is disposed as an exposure unit that irradiates light based on an image signal to form an electrostatic latent image on the photosensitive drum 10. A predetermined portion on the photosensitive drum 10 is exposed by the exposure head 30, and charging of the predetermined portion of the photosensitive drum 10 is attenuated.

  Further, a developing unit 20 as a developing unit that develops an electrostatic image as a toner image is disposed around the photosensitive drum 10. In the present embodiment, the developing unit 20 uses a non-magnetic one-component developer, that is, toner, as the developer. In the present embodiment, the developing unit 20 performs development by bringing a developing roller 21 as a developer carrying member into contact with the photosensitive drum 10. That is, in the present embodiment, a voltage having the same polarity as the charging polarity of the photosensitive drum 10 (negative polarity in the present embodiment) is applied to the developing roller 21 in the developing unit 20. For this reason, an electric field is generated between the developing roller 21 and the photosensitive drum 10 connected to the ground, and the negatively charged toner is a portion where the charge is attenuated by exposure on the photosensitive drum 10 (image portion, The electrostatic image is developed by adhering to the exposed portion.

  Further, an intermediate transfer belt 50 as an intermediate transfer body (transfer material) for transferring the toner image on the photosensitive drum 10 to the recording paper 100 is disposed so as to face the four photosensitive drums 10. . Here, the intermediate transfer belt 50 formed of an endless belt as an intermediate transfer member abuts on all the photosensitive drums 10 and circulates (rotates) in the direction indicated by the arrow (counterclockwise). The intermediate transfer belt 50 is stretched around a plurality of support members, a primary transfer roller 51, a secondary transfer counter roller 55, a driven roller 53, and a driving roller 54. On the inner peripheral surface side of the intermediate transfer belt 50, four primary transfer rollers 51 as primary transfer portions are arranged in parallel so as to face the respective photosensitive drums 10. The primary transfer roller 51 presses the intermediate transfer belt 50 toward the photosensitive drum 10 to form a primary transfer portion where the intermediate transfer belt 50 and the photosensitive drum 10 come into contact with each other. A bias having a polarity opposite to the charging polarity of the toner is applied to the primary transfer roller 51 from a primary transfer bias power source (high voltage power source) as a primary transfer bias application unit (not shown). As a result, the toner image on the photosensitive drum 10 is transferred (primary transfer) onto the intermediate transfer belt 50.

  Further, a secondary transfer roller 52 as a secondary transfer portion is disposed at a position facing the secondary transfer counter roller 55 on the outer peripheral surface side of the intermediate transfer belt 50. The secondary transfer roller 52 is pressed against the secondary transfer counter roller 55 via the intermediate transfer belt 50 to form a secondary transfer portion where the intermediate transfer belt 50 and the secondary transfer roller 52 come into contact with each other. Then, a bias having a polarity opposite to the normal charging polarity of the toner is applied to the secondary transfer roller 52 from a secondary transfer bias power source (high voltage power source) as a secondary transfer bias application unit (not shown). As a result, the toner image on the intermediate transfer belt 50 is transferred (secondary transfer) to the recording paper 100 fed from the paper feeding unit.

  Further, a cleaning unit 90 for cleaning toner remaining on the surface of the photosensitive drum 10 after transfer (transfer residual toner) is disposed.

  In this way, charging, exposure, development, transfer, and cleaning are performed in the order of rotation of the photosensitive drum 10.

  Finally, the recording paper 100 onto which the toner image has been transferred is conveyed to a fixing device 80 as a fixing unit. The toner image is fixed on the recording paper 100 by applying heat and pressure to the recording paper 100 in the fixing device 80.

  The secondary transfer residual toner remaining on the intermediate transfer belt 50 after the secondary transfer process is cleaned by the intermediate transfer belt cleaning device 56.

  The image forming apparatus 1 can also form a single-color or multi-color image using only a desired single or some (not all) image forming units.

  The image forming apparatus configuration described so far is merely an example for explaining the present embodiment, and is not limited to the gist of the present invention.

  Next, the exposure head 30 will be described. FIG. 2A shows an assembly diagram of the exposure head 30, and FIG. 2B shows an exploded view of the exposure head 30. As shown in FIG. 2B, the exposure head 30 includes an element array 301 in which a plurality of organic EL elements are arranged, a frame 360, and a lens group 310. The element array 301 and the lens group 310 are aligned at an appropriate position determined by the focal position of the lens group 310, and are positioned and fixed to the frame 360.

  A detailed view of the element array 301 is shown in FIG. 3, the element array 301 includes a substrate 305, a plurality of organic EL elements 302 arranged on the substrate 305, a drive circuit 303 for driving the plurality of organic EL elements 302, and a connector 304. . Specifically, the drive circuit 303 and the plurality of organic EL elements 302 are monolithically formed on the substrate 305.

  In this embodiment, the organic EL element 302 is a bottom emission type, and light is emitted through the substrate 305. The plurality of organic EL elements 302 are sealed with a sealing member (not shown). As shown in FIG. 3, the plurality of organic EL elements 302 are arranged on the substrate 305 in a staggered pattern in the Y direction. By controlling the light emission timing of each organic EL element 302 by the drive circuit 303, the spots exposed by each organic EL element 302 are arranged in a straight line on the photosensitive drum 10. At this time, the arrangement position of the organic EL element 302 and the position and shape of the lens group 310 are designed so that the spots on the photosensitive drum 10 slightly overlap each other. The organic EL elements 302 may be arranged in a line without being arranged in a staggered pattern.

  The connector 304 is electrically connected to the drive circuit 303 by wiring, and is connected to a control board (not shown) on the main body side of the image forming apparatus (not shown) by a cable. The organic EL element 302 emits light according to a data signal input from the control board on the main body side via the connector 304. Specifically, the current value flowing through each organic EL element 302 is controlled by the drive circuit 303 in accordance with the image signal, thereby causing the organic EL element 302 to selectively emit light with a desired luminance.

  FIG. 4 shows a TFT circuit (transistor circuit) 306 constituting the drive circuit 303 shown in FIG. A plurality of TFT circuits 306 here are circuits for driving one organic EL element 302 and controlling the light emission. The drive circuit 303 has the number of TFT circuits 306 of organic EL elements. Note that the signal line, the reference voltage line, and the power supply line are commonly connected to the TFT circuits 306.

  The TFT circuit 306 has five TFT elements. Then, by connecting the five TFT elements as shown in FIG. 4, the value of the current flowing through the organic EL element 302 is controlled in accordance with the data signal sent through the signal line. The organic EL element 302 emits light with a luminance corresponding to the current value supplied from the TFT circuit. With such a configuration, the organic EL element 302 emits light with luminance corresponding to the image signal. As shown in FIG. 4, the TFT circuit 306 of the present embodiment flows in luminance (hereinafter referred to as amplitude) per unit time of the organic EL element 302, specifically, the organic EL element 302 for gradation expression. In this configuration, the current value is controlled. This configuration is called amplitude modulation.

  On the other hand, a TFT circuit 316 (a portion surrounded by a thick line) shown in FIG. 5 has a configuration for controlling the light emission time of the organic EL element 302 as in a general laser scanner. This configuration is called pulse width modulation. As shown in FIG. 5, the TFT circuit 316 requires a constant current source circuit similar to the configuration of the TFT circuit 306 shown in FIG. In addition, the TFT circuit 316 needs an EV-PWM circuit and an OD-PWM circuit in order to drive the constant current source circuit in a time division manner. For this reason, the circuit scale of the TFT circuit 316 for performing pulse width modulation increases. Specifically, the number of TFT elements in the TFT circuit 316 is 21 times, which is about four times that of the TFT circuit 306 shown in FIG.

  That is, the TFT circuit 306 that performs amplitude modulation as in the present embodiment can reduce the number of TFT elements compared to the TFT circuit 316 that performs pulse width modulation. Therefore, according to this embodiment, the area where the drive circuit 303 is formed can be reduced, and the area of the substrate 305 can be reduced. If the area of the substrate 305 is reduced, the number of element arrays 301 that can be taken from one large substrate can be increased, and the cost can be reduced.

  Further, when the LED element is used as a light emitting element, the luminance per unit time of the organic EL element 302 is controlled on the same substrate as the element array in which a plurality of light emitting elements are arranged as in this embodiment. It is difficult to form a transistor circuit. This is because the substrate on which the LED element is formed is generally a GaAs substrate or a GaN substrate, so that it is difficult to configure a transistor circuit. For this reason, when an LED element is used as a light emitting element, an external circuit such as a main controller or a head controller described later becomes large. On the other hand, when the organic EL element 302 and the drive circuit 303 (TFT circuit 306) are formed on the same glass substrate or Si substrate as in the present embodiment, the external circuit can be reduced and the cost can be reduced. be able to.

  Next, in order to describe a data signal input when the organic EL element 302 emits light, a processing path of an image signal input from the outside is shown in FIG. An image signal 350 input from an external device such as a host computer is held in a main controller 351 having a CPU and a memory. Thereafter, the main controller 351 sends a control signal for moving the image forming apparatus 1 and gives an image signal 350 to the head controller 352. The head controller 352 refers to the light amount correction data in the correction memory 353 and converts the image signal 350 into a multi-value area gradation signal corresponding to the exposure heads 30K, 30C, 30M, and 30Y for each color by the area gradation method. Convert and create. Thereafter, taking into account that the organic EL elements 302 are in a staggered arrangement, a process for adjusting the signal writing order is performed so as to be exposed on a straight line on the photosensitive drum 10 and arranged for each color. An area gradation signal is transmitted to the exposure heads 30K, 30C, 30M, and 30Y. Based on the area gradation signal, the drive circuit 303 sends a data signal corresponding to each organic EL element to the signal line connected to each TFT circuit 306, and based on the data signal, the unit circuit per unit time of each organic EL element. Control brightness.

  Next, the image forming apparatus of the present embodiment performs halftone image gradation expression by combining a binary area gradation method and an amplitude modulation method as a multi-value area gradation method. Hereinafter, the gradation expression of the present embodiment will be described. Note that the binary area gray scale method and the amplitude modulation method are obtained by comparing the gray scale expression combining the binary area gray scale method and the pulse width modulation method as the multi-value area gray scale method. The effect of the combination is also explained.

  First, with reference to FIG. 7, description will be given of a gradation expression that combines a binary area gradation method and a pulse width modulation method. In FIG. 7B, nine block elements existing in the unit cell 600 of the electrostatic latent image formed by the exposure head are referred to as pixels (601 to 609). In this way, using a unit cell 600 of 3 × 3 (corresponding to 600 dpi of 200 lines) as one unit, a halftone image as shown in FIG. Note that the three pixels arranged in the column direction in the unit cell 600 correspond to the exposure position of the same organic EL element 302, and the positions of exposure on the photosensitive drum 10 differ according to the rotation of the photosensitive drum 10. Yes. Further, twelve pixels arranged in the same column in FIG. 7A also correspond to the exposure position of the same organic EL element 302.

  In addition, the pixel 605 in the unit cell 600 is a pixel that performs pulse width modulation, and is referred to as a gradation control pixel. However, the gradation control pixel is at least one pixel selected from the pixels in the unit cell 600, and is appropriately determined by a dither method or an error diffusion method that is an area gradation method.

  The black part in FIG. 7B is an exposed part, and the white part is a non-exposed part. That is, in the unit cell 600 of FIG. 7B, the pixels 601, 602, and 604 are in an exposed state, and the pixels 603, 606, 607, 608, and 609 are not in an exposed state. Further, the pixel 605 is in an exposure state in which a part of the pixel, specifically, the center of the pixel 605 spreads in the vertical direction of the paper surface, and the other part is not exposed.

  In the case of a configuration that does not have a gradation control pixel, that is, when a gradation is expressed only by a binary area gradation method, one pixel has a binary pattern in an exposed state and a non-exposed state. In the unit cell, only 3 × 3 = 9 gradations can be expressed.

However, when the unit cell 600 has the gradation control pixel 605 as described above, the following light emission control is possible. That is, in addition to the exposure state and the non-exposure state, an intermediate state in which a part is exposed and the other part is not exposed can be created for each pixel. As a result, for example, a 4-bit data signal can be assigned to the organic EL element 302 corresponding to each pixel, and a gradation expression of 3 × 3 × 2 ⁴ = 144 gradations can be obtained by controlling the exposure time. Can do.

  The gradation control pixel 605 will be described in more detail with reference to FIG. FIG. 8A shows an exposure state corresponding to the area gradation signal of the unit cell 600 at gradation 48 out of 144 gradations. When the gradation is increased by two from the area gradation signal shown in FIG. 8A, the state shown in FIG. 8B is obtained. Specifically, FIG. 8B shows the exposure time of the gradation control pixel 605 when the exposure time corresponding to the maximum gradation value that can be expressed only by that pixel is 1 (= 16/16). This is a case where it is increased by 2/16 compared to FIG. FIG. 8B shows an exposure state in which the unit cell 600 corresponds to 50 gradations. Similarly, a state in which the gradation is increased by two gradations in the order of FIGS. That is, FIGS. 8A to 8I show halftone image patterns for gradations 48, 50, 52, 54, 56, 58, 60, 62, and 64 in order.

  The gradations 65 to 80 can be expressed by controlling the exposure time using the pixel 603 as a gradation control pixel in FIG. In this way, gradations from 1 to 144 can be expressed.

  FIG. 8 shows a center growth type pulse width modulation method in which the exposure time is controlled so as to spread from the center of the gradation control pixel 605 toward both ends. In addition, there is an edge growth type pulse width modulation method in which the exposure time is controlled so as to spread from one end of the gradation control pixel 605 toward the other end.

  Next, with reference to FIG. 9, description will be given of gradation expression combining the binary area gradation method and the amplitude modulation method. Similarly to the method using the pulse width modulation method described above, in FIG. 9B, nine block elements existing in the unit cell 700 of the electrostatic latent image formed by the exposure head 30 are pixels (701 to 709). The gradation of one unit cell 700 is expressed. Then, a halftone image as shown in FIG. 9A is formed by a plurality of groups of unit cells 700. The three pixels arranged in the column direction in the unit cell 700 correspond to the exposure position of the same organic EL element 302, and the exposure position on the photosensitive drum 10 is different in accordance with the rotation of the photosensitive drum 10. Yes. Further, twelve pixels arranged in the same column in FIG. 9A also correspond to the exposure position of the same organic EL element 302.

Further, by providing the gradation control pixel 705 in the unit cell 700, the number of gradations that can be expressed is increased. Specifically, in the gradation control pixel 705, the current amount of the organic EL element 302 corresponding to the gradation control pixel 705 is controlled so that the light emission luminance value per unit time can take a plurality of values. . With this configuration, the gradation control pixel 705 takes a plurality of values for luminance per unit time. Even in this configuration, a 4-bit data signal can be assigned to the organic EL element 302 corresponding to each pixel, and by controlling the exposure time, 3 × 3 × 2 ⁴ = 144 gradations corresponding to 600 dpi of 200 lines. A gradation expression can be obtained.

  FIG. 9B shows an exposure state corresponding to 56 area gradation signals of the gradation 144. In FIG. 9B, the black portions are portions exposed at the maximum luminance value, and the gray portions are portions exposed at half the maximum luminance value. Thus, unlike the pulse width modulation method, the gradation control pixel 705 is not in a state where the exposure state and the non-exposure state are mixed, and the entire gradation control pixel 705 is in the exposure state, but the exposure amount is the pixel 701. It is smaller than The luminance per unit time of the device EL element 302 corresponding to the gradation control pixel 705 in the unit cell 700 is set to a value other than the maximum luminance or the minimum luminance by the driving circuit 303. On the other hand, the luminance per unit time of the organic EL element 302 corresponding to a pixel (for example, 701 or 709) that is not the gradation control pixel 705 is set by the drive circuit 303 to a maximum luminance value or a minimum luminance value.

  However, like the pulse width modulation method, the gradation control pixel 705 is at least one pixel selected from the pixels in the unit cell 700, and is determined by a dither method or an error diffusion method that is an area gradation method. The

  The gradation control pixel 705 will be described in detail with reference to FIG. FIG. 10A shows an exposure state corresponding to the area gradation signal of the unit cell 700 at the gradation 48. When the gradation is increased by two from the area gradation signal shown in FIG. 10A, the state shown in FIG. Specifically, FIG. 10B shows the amplitude value of the gradation control pixel 705 when the maximum amplitude value corresponding to the maximum gradation value that can be expressed only by that pixel is 1 (= 16/16). This is a case where it is increased by 2/16 compared to FIG. Then, the gradation of the unit cell 700 becomes 50 gradations. Similarly, it represents a state in which two gradations are increased in the order of FIGS. That is, FIGS. 10A to 10I show halftone image patterns for gradations 48, 50, 52, 54, 56, 58, 60, 62, and 64 in order.

  The gradations 65 to 80 can be expressed by controlling the exposure amount (amplitude value) of the pixel 703 as a gradation control pixel in FIG. In this way, gradations from 1 to 144 can be expressed.

  As a multi-value area gradation method, exposure simulation results in the case of combining a binary area gradation method with a center growth type pulse width modulation method or an amplitude modulation method are compared.

  FIG. 11 shows a simulation result of the exposure distribution cross section in the BB cross section of the unit cell 600 of FIG. 8 when the exposure is performed according to the gradation pattern using the method shown in FIG. In FIG. 11, the horizontal axis represents position, and the vertical axis represents luminance. Lines 630 to 638 in FIG. 11 correspond to the exposure distribution cross sections in FIGS. 8A to 8I in order.

  On the other hand, FIG. 12 shows the simulation result of the exposure distribution section in the CC section of the unit cell 700 of FIG. 10 when the exposure is performed according to the gradation pattern using the method of the present embodiment shown in FIG. Lines 730 to 738 in FIG. 12 correspond to the exposure distribution cross sections in FIGS. 10A to 10I in order.

  The above exposure simulation is a calculation of an exposure distribution as an output when a spot shape after passing through a lens group 310 described later is input corresponding to an arbitrary area gradation signal. Specifically, the exposure image pattern based on the input spot shape and the area gradation signal is subjected to fast Fourier transform and convolution. The spot shape at the time of input is standardized by the integrated light quantity per unit pixel at the time of solid exposure. The simulation was performed by setting the light emitting region of the organic EL element 302 to 42 μm × 42 μm.

  By comparing FIG. 11 and FIG. 12, the multi-value area gradation method using the amplitude modulation method of the present embodiment is more effective than the multi-value area gradation method using the center growth type pulse width modulation method. It can be seen that an image that is stable against environmental fluctuations is formed. The reason is described below.

  In FIG. 11 and FIG. 12, a change in exposure distribution at a predetermined luminance value serving as a threshold will be described. An asterisk (*) indicates an intersection of the curve 630 in FIG. 11 and the curve 730 in FIG. 12 at luminance values of 0.25, 0.5, and 0.75. Similarly, the intersections of the curves 631 to 638 in FIG. 11 and the curves 731 to 738 in FIG. 12 and the luminance values 0.5, 0.25, and 0.75 are indicated by circles (◯) and squares ( □), indicated by a triangle (Δ). Using the distance between each mark (◯, □, Δ) and the ☆ mark as the amount of change in line width, the amount of change in line width is evaluated for each gradation. The reason why the distance between the mark ☆ and the marks (◯, □, Δ) in FIGS. 11 and 12 is defined as the amount of change in line width will be described below.

  In the electrophotographic image forming apparatus, an electrostatic latent image is formed on the photosensitive drum 10 by irradiating light based on an image signal. For this reason, if the exposure distribution on the irradiated photosensitive drum 10 changes, the latent image potential on the photosensitive drum 10 also changes. In view of the change in the exposure distribution, the change in the latent image potential on the photosensitive drum 10 can be predicted. The luminance value 0.5 of the exposure distribution shown here corresponds to the voltage value applied to the developing roller 21 described above. When a portion having a luminance value greater than 0.5, that is, a curve corresponding to the exposure distribution cross section is above the one-dot chain line in FIGS. 11 and 12, the toner adheres to the exposure position and becomes a development portion. . When a portion having a luminance value smaller than 0.5, that is, a curve corresponding to the exposure distribution cross section is below the one-dot chain line in FIGS. 11 and 12, no toner is attached to the exposure position, Become. That is, the luminance value is a threshold value for the development portion and the non-development portion. Therefore, it is possible to evaluate the change in line width (the distance between ☆ and ○ in FIGS. 11 and 12) at the intersection of the luminance value as a threshold and the curve corresponding to the exposure distribution section as the change in image. The stability of gradation expression can be known from the change rate of the change amount.

  Also, the luminance value 0.75 and the luminance value 0.25 represent the development bias fluctuation in a high temperature and high humidity environment or a low temperature and low humidity environment. By comparing the variation of the line width change amount (distance between ☆ and □ or Δ in FIGS. 11 and 12) at the luminance value 0.75 or 0.25, the gradation expression under the environmental variation Can be evaluated.

  FIG. 13 shows a gradation using amplitude modulation or a gradation of pulse width modulation of the center growth type and a line width change amount. The dotted line shows the relationship between the line width change amount and the gradation in the center growth type pulse width modulation shown in FIG. More specifically, the relationship between the line width change amount and the gradation when the luminance value is 0.5 is a dotted line 650, and the relationship between the line width change amount and the gradation when the luminance value is 0.75 is a dotted line 651. The relationship between the line width change amount and the gradation when the luminance value is 0.25 is indicated by a dotted line 652. On the other hand, the solid line shows the relationship between the line width change amount and the gradation in the amplitude modulation shown in FIG. More specifically, the relationship between the line width change amount and the gradation when the luminance value is 0.5 is a solid line 750, and the relationship between the line width change amount and the gradation when the luminance value is 0.75 is a solid line 751. A solid line 752 indicates the relationship between the line width change amount and the gradation when the value is 0.25.

  As shown in FIG. 13, between the dotted line 650 and the solid line 750, the solid line 750 has a smaller change in inclination (change in line width change amount) and is relatively close to a constant inclination. For this reason, when the amplitude modulation method is used at a luminance value of 0.5, more stable gradation expression can be performed than when the pulse width modulation method is used.

  Further, when the luminance value becomes 0.25 or 0.75 due to the environmental change, a difference due to the difference in gradation expression occurs. In the center-growth type pulse width modulation, the difference in the line width change amount at the luminance values of 0.25 and 0.75 is the largest at the gradation 54, and the difference value is 29 μm. On the other hand, the amplitude modulation is 21 μm at the gradation 56. This result means that the amplitude modulation has a smaller amount of change in the line width with respect to the variation of the threshold luminance value than the center growth type pulse width modulation. That is, it can be said that amplitude modulation is a method capable of expressing gradation more stably than pulse width modulation with respect to environmental changes.

  Also, the dotted line 651 in FIG. 13 has no change in the line width from the gradation 48 to 54. Then, a non-zero value of the line width change amount from the gradation 56 is taken. For this reason, when the luminance value of the threshold value becomes 0.75 due to the environmental change, the gradations from gradations 48 to 54 are developed with gradation 48, and the gradation is lost. A jump occurs and a defect occurs in the formed image. On the other hand, the solid line 751 has a change in the line width even between the gradations 48 and 54, and the gradation is not lost. Also in this respect, an image can be stably formed when amplitude modulation is used as compared with the case where center growth type pulse width modulation is used with respect to environmental fluctuations.

  This is because the curve corresponding to the exposure distribution section of the electrostatic latent image for each gradation by the center growth type pulse width modulation shown in FIG. 11 has a stepped portion. The line width changes abruptly at this stepped portion. As can be seen from the square mark on the curve 632 (tone 52 curve) in FIG. 11, the distance is larger than the square mark on the curve 631 (tone 50 curve). This is because the curve changes greatly at the position of the stepped portion 639 of the curve 632 (curve of gradation 52) and spreads to the right in FIG. In FIG. 11, the curve 631 to the curve 636 have a stepped portion in a range where the luminance is less than 1 greater than 0, and thus there is a portion where the line width change amount changes greatly when the threshold value changes. For this reason, the tone jump occurs due to the setting of the threshold value or the environmental change. On the other hand, as shown in FIG. 12, in amplitude modulation, none of the curves corresponding to the exposure distribution cross section has a stepped portion, so that a tone jump hardly occurs.

  Here, as an example, gradations from 48 to 64 are shown, but it has been confirmed that amplitude modulation is more stable than pulse width modulation in all other gradations. is there.

  As described above, by combining amplitude modulation with binary area gradation as the gradation expression of the exposure head 30, it is possible to realize an image forming apparatus with low cost and high stability against environmental fluctuations.

  Next, the lens group 310 according to the present embodiment will be described. FIG. 14 shows the configuration of the lens group 310 according to this embodiment. The lens group 310 includes a first lens array 320 and a second lens array 340 arranged in the X direction. The second lens array 340 includes a first lens row 343 and a second lens row 344 arranged in the Z direction. The first lens array 320 has the same configuration. Each of the first lens array 343 and the second lens array 344 has a plurality of lenses arranged in the Y direction.

  In the present embodiment, the X direction is referred to as an optical axis direction, the Y direction is referred to as a main arrangement direction, and the Z direction is referred to as a sub arrangement direction. The main arrangement direction is parallel to the longitudinal direction in which the organic EL elements 302 are arranged one-dimensionally in the element array 301. The sub array direction is a direction corresponding to the rotation direction of the photosensitive drum 10.

  In addition, a plurality of light shielding members 330 are arranged between the first lens array 320 and the second lens array 340. The light blocking member 330 passes a part of the light beam that passes through the lenses in the first lens array 320 and enters the lenses in the second lens array 340 (stray light that does not contribute to image formation) in the main array section. Plays the role of shading.

  An array (optical axis array) formed by optical axes of a plurality of lenses included in the second lens array 340 is between the first lens array 343 and the second lens array 344 in the second lens array 340. It is located on the same line included in a certain plane. The first lens array 320 has the same configuration. Furthermore, the rows (optical axis rows) formed by the optical axes of the plurality of lenses included in the first lens array 320 are the first lens row 343 and the second lens row 344 in the second lens array 340. It is also located on the same line included in the plane between. As a result, a staggered array (staggered array) is realized with a system that forms an inverted image of an object in a ZX section that is a section perpendicular to the main array direction (hereinafter referred to as a main array section). . Hereinafter, an erecting equal magnification imaging system is an erecting equal magnification imaging system, and an inverted imaging system is an inverted imaging system.

  The “stage shift arrangement (staggered arrangement)” in this embodiment is defined as follows. That is, in a configuration in which one lens array has a plurality of lens rows, the optical axes of the plurality of lenses included in each of the plurality of lens rows do not coincide with each other in the sub-array direction, and the main array direction It is the structure which is spaced apart and is located on the same line. Here, the lenses adjacent to each other in the sub-array direction refer to the lenses closest to each other in the sub-array direction. That is, the term “adjacent” includes a configuration in which lenses arranged in the sub-array direction are in contact with each other and a configuration in which the lenses are arranged via an intermediate.

  Next, the lens group 310 used in the present invention will be described in detail with reference to specific numerical values with reference to FIGS. FIGS. 15A to 15C are schematic views of main parts of the lens group 310 according to the present embodiment. 15A to 15C respectively show an XY cross section, a ZX cross section (main array cross section), and a YZ cross section (hereinafter referred to as a sub array cross section) of the lens group 310.

  As shown in FIG. 15A, the light beam emitted from one light emitting point on the element array 301 and passing through each lens is condensed on one point on the photosensitive drum 10. For example, the light beam from the light emitting point P1 on the element array 301 is focused on P1 ′, and the light beam from the light emitting point P2 is focused on P2 ′. With this configuration, the light emitting point on the element array 301 emits light. Exposure corresponding to the state becomes possible. Note that the light emitting points on the element array 301 have a plurality of light emitting points arranged at equal intervals in the main array direction, and the interval between adjacent light emitting points is several tens of μm. That is, since the distance between adjacent light emitting points is sufficiently smaller than the distance between adjacent lenses (several hundreds μm or more) in the main array direction, it can be considered that each light emitting point exists substantially continuously hereinafter.

  Each light emitting point on the element array 301 forms an erecting equal-magnification image in the main array section shown in FIG. 15A, and an inverted image in the sub-array section shown in FIG. 15B. . Further, as shown in FIG. 15C, each lens array (for example, the second lens array 320) has an upper row (first lens row 343) and a lower row (second lens row) in the sub-array direction. 344) has two lens rows. The optical axes of the lenses included in the upper row are indicated by black circles (●), and the optical axes of the lenses included in the lower row are indicated by inverted triangle marks (▽). The arrangement pitch p in the main arrangement direction of each lens in the lens row is 0.76 mm for both the upper row and the lower row.

  Here, as shown in FIG. 15C, each optical axis in the upper row and each optical axis in the lower row are located on the same line 345 (optical axis row). If this same line is Z = 0, there is a lens surface in the lower row in the range of Z = −1.22 mm to 0 mm, and a lens surface in the upper row in the range of Z = 0 mm to 1.22 mm. Further, by shifting the upper row and the lower row from each other by ΔY in the main arrangement direction, the respective optical axes are spaced apart in the main arrangement direction and arranged in a staggered manner. Here, the shortest distance ΔY between the optical axis of the upper row and the optical axis of the lower row is based on the optical axis of one lens of the lower row in the main arrangement direction, and the light of the upper row lens closest to the optical axis. The shortest distance to the axis. In the present embodiment, the shortest distance ΔY is half of the arrangement pitch p in the main arrangement direction of the lenses, and ΔY = p / 2 = 0.38 mm.

In addition, each of the entrance surface and exit surface (321, 322, 341, 342) of each lens of the first lens array 320 and the second lens array 340 shown in FIG. 15 is formed of an anamorphic aspheric surface. Here, the intersection of each lens surface of the lens array and the optical axis (X axis) is the origin, the axis orthogonal to the optical axis in the main array direction is the Y axis, and the axis orthogonal to the optical axis in the sub array direction is the Z axis. Then, the shape SH of the anamorphic aspheric surface is represented by the following expression (1). Here, C _{i, j} (i is an integer of 0 or more, j is an integer of 0 or more) is an aspheric coefficient.

SH = ΣC _{i, j} Y ⁱ Z ^j (1)
Table 1 shows optical design values of the respective lenses. In Table 1, G1 indicates a lens included in the first lens array 320, and R1 indicates a point where the light incident surface of the lens intersects with the optical axis of the lens. R2 indicates the point where the light exit surface of the lens and the optical axis of the lens intersect. That is, G1R1 represents a point where the light incident surface 321 of the lens included in the first lens array 320 and the optical axis of the lens intersect. G1R2 represents a point where the light exit surface 322 of the lens included in the first lens array 320 and the optical axis of the lens intersect. The same applies to G2R1 and G2R2.

  As shown in Table 1, in this embodiment, the intermediate imaging magnification β (details will be described later) in the main array section of each lens is set to −0.45, but it is upright in the main array direction. As long as the optical system is in the same magnification range, β can take any value.

  FIG. 16A is a main array cross-sectional view of an upper-row lens optical system including an upper row 323 of the first lens array 320 and an upper row (first lens row 343) of the second lens array; A sub-array sectional view is shown. On the other hand, FIG. 16B shows a main array of lens optical systems in the lower row composed of the lower row 324 of the first lens array 320 and the upper row (second lens row 344) of the second lens array. Sectional drawing and subarray sectional drawing are shown.

  As can be seen by comparing FIGS. 16A and 16B, the lens optical system in the upper row and the lens optical system in the lower row have the same configuration in the main array section, and the light beam paths are also the same. On the other hand, in the sub-array cross section, these lens optical systems have a symmetric configuration with respect to the optical axis. The lens optical systems constituting the upper row and the lower row are respectively arranged on the same optical axis, the first optical system (lens of the first lens array 320) and the second optical system (second lens array 340). Lens). Here, an optical system that forms an intermediate image of each light emitting point on the element array 301 is a first optical system, and a surface on which the first optical system forms an intermediate image is an intermediate imaging surface 17. In addition, an optical system that forms an intermediate image formed on the intermediate imaging surface 17 on the photosensitive drum 10 is a second optical system. In the present embodiment, the first optical system is configured by only the lenses in the first lens array 320, and the second optical system is configured by only the lenses in the second lens array 340.

  Next, the effect accompanying the lens will be added below. First, the effect of the staggered arrangement (staggered arrangement) that is the configuration of the lens array of the present embodiment will be described. For comparison, consider a lens array optical system in which a lens array does not have a plurality of columns in the sub-array direction and is arranged in only one column. In the comparative example, the configuration (optical design values, etc.) other than the above is the same as that of the lens group according to the present embodiment.

  FIG. 17 is a diagram showing a sub-array cross section of the lens group of the comparative example, and FIGS. 17A to 17C show the state of an imaging light beam made up of light beams emitted from different light emission positions. As shown in FIG. 17A, the imaging light beam from the light emission position A is composed only of a lens light beam having an object height of 0 in one lens optical system. As shown in FIG. 17B, the imaging light beam from the light emitting point position B has a lens light beam having an object height p / 4 of one lens optical system and an object height 3p / 4 of the lens optical system adjacent thereto. It consists of a lens beam. As shown in FIG. 17C, the imaging light beam from the light emission point position C is composed of two lens light beams having an object height p / 2 of two adjacent lens optical systems. As described above, in the comparative example, since the lens light beam constituting the imaging light beam at each light emitting point position is small, the light amount difference between the light emitting point positions of one lens light beam greatly affects the difference in the imaging light amount. .

  FIG. 18 is a diagram illustrating a sub-array cross section of the lens group of the present embodiment, and FIGS. 18A to 18C are respectively obtained by light beams emitted from the same light emitting positions as those in FIGS. 17A to 17C. The state of the imaging light flux is shown. According to the lens group according to the present embodiment, the first lens array 320 and the second lens array 340 are applied with a staggered arrangement, so that the number and types of lens light beams constituting the imaging light beam (the object height) Difference) can be increased. As a result, it is possible to average the imaging light flux at each light emitting point position, and an effect of reducing imaging light amount unevenness and imaging performance unevenness can be obtained. In particular, in the present embodiment, ΔY = p / 2, so that the imaging light fluxes at the light emission point position A and the light emission point position C can be made equal.

  Next, in order to evaluate the imaging light amount unevenness, the ratio of the imaging light amount corresponding to each light emitting point position is shown in FIGS. FIG. 19 shows the ratio of the imaged light quantity when the lens group of the comparative example is used, and FIG. 20 shows the ratio of the imaged light quantity when the lens group of the present embodiment is used. The imaging light quantity at each light emitting point position is proportional to the integration of the light utilization efficiency of the lens light beam constituting the imaging light beam, and the imaging light quantity at the light emitting point position on the optical axis of a certain lens optical system is set to 100. Normalized as%.

  As shown in FIG. 19, in the comparative example, a difference between the maximum value and the minimum value of the imaging light amount ratio is 6.2%. On the other hand, as shown in FIG. 20, in this embodiment, the difference between the maximum value and the minimum value of the imaging light quantity ratio is within 1.0%. In other words, it can be seen that the lens group according to the present embodiment can reduce the unevenness of the imaged light quantity more than the lens group of the comparative example.

  As shown in FIG. 15, the light beam emitted from the light emitting point on the element array 301 in the main array cross section passes through the first lens array 320 and then forms an intermediate image on the intermediate imaging surface 17. Then, an erecting equal-magnification image is formed on the photosensitive drum 10 through the second lens array 340. At this time, the paraxial imaging magnification at the intermediate imaging surface 17 of the first lens array 320 is defined as an intermediate imaging magnification β. On the other hand, in the sub-array section, the light beam emitted from the light emitting point on the element array 301 passes through the first lens array 320 and then passes through the second lens array 340 without being subjected to intermediate imaging. An inverted image is formed on the body drum 10. As described above, the lens group 310 according to the present embodiment is an inverted imaging system in the sub-array direction, so that the light capture angle can be increased while maintaining the imaging performance, and the imaging light amount and the coupling amount can be increased. Both image performance is achieved.

  In addition, although the lens group 310 showed the example comprised by two lens arrays of the 1st lens array 320 and the 2nd lens array 340, it is not restricted to this. That is, the lens group 310 may have a configuration including three or more lens arrays arranged in the X direction. In this case, as described above, at least one of the first optical system and the second optical system may be configured by two lenses. However, since the number of parts increases when each lens array is composed of three or more, it is preferable that the lens group 310 is composed of two lens arrays.

  Further, each of the lens optical systems constituting the lens group 310 may be configured by one lens array without being divided into the first optical system and the second optical system. Even in such a case, the above-described effects can be obtained by configuring one lens array to be an erecting equal magnification imaging system in the main array section and an inverted imaging system in the sub array section. Think to play.

  In the present embodiment, the shape of the lens constituting the upper row and the shape of the lens constituting the lower row correspond to the respective shapes obtained by cutting and dividing one lens optical system along the main array section including the optical axis. It has become the composition. That is, in the main arrangement direction, when the shortest distance ΔY to the optical axis of the upper stage lens closest to the optical axis when the optical axis of the lower stage lens is used as a reference is assumed to be 0 (arranged in a staggered manner) The lens surfaces of the adjacent upper row lenses and lower row lenses are configured to have a shape that can be expressed by the same expression. Even when the upper row and the lower row are arranged via an intermediate, molding can be facilitated if the lens surfaces have shapes that can be expressed by the same expression.

  As shown in FIG. 15, the first optical system (the lenses of the first lens array 320) and the second optical system (of the second lens array 340) for the upper and lower lens optical systems. Lens) is symmetrical with respect to the intermediate image plane 17. With this configuration, the same member can be used for both optical systems. Moreover, it is desirable that the apertures of all the lens surfaces constituting the lens group 310 have a rectangular shape. That is, by making the aperture surfaces of the on-axis object high light fluxes of the first optical system and the second optical system rectangular, the lens surfaces can be arranged with no gap as much as possible, and the light utilization efficiency can be improved. In addition, the rectangular shape here includes a shape in which at least one of the sides constituting the rectangle is curved, a shape in which each vertex is eliminated, and a substantially circular shape or a substantially elliptical shape. Yes.

  In the present embodiment, the configuration in which the optical axes of all the lenses in each lens row are located on the same line has been described. Here, when the size of each light emitting point of the light emitting unit in the image forming apparatus in the sub array direction is H, and the maximum distance in the sub array direction between the optical axis arrays of each lens array is Δ, the following conditional expression (2 ), It is defined that each optical axis is located on the same line.

Δ <(1/2) H (2)
If the distance between the optical axes in the sub-array direction is within the range of the conditional expression (2), the images of the lens rows are not separated from each other, so that the effects of the present invention can be sufficiently obtained. The size H in the sub-array direction of each light emitting point of the light emitting part is 25.3 μm. Therefore, in all the lenses, if the maximum separation amount Δ in the sub-array direction between the optical axes is smaller than (1/2) H = (1/2) × 42.3 μm = 21.7 μm, The effects of the invention can be sufficiently obtained.

DESCRIPTION OF SYMBOLS 10 Photoconductor 20 Developing part 30 Exposure part 40 Charging part 80 Fixing part 301 Element array 302 Organic EL element 303 Drive circuit 306 TFT circuit 310 Lens group

Claims (8)

A photosensitive member, a charging unit that charges the photosensitive member, an exposure unit that forms an electrostatic latent image on the surface of the photosensitive member, a developing unit that develops the electrostatic latent image as a toner image, and the toner image An image forming apparatus comprising: a transfer unit that transfers to a transfer material; and a fixing unit that fixes the transferred toner image to the transfer material;
The exposure unit includes a lens group having a plurality of lenses arranged in a first direction, and a plurality of organic elements arranged opposite to the lens group and arranged in parallel with the first direction on a substrate. An element array having an EL element,
A drive circuit having a plurality of transistor circuits for controlling the luminance per unit time of the plurality of organic EL elements is disposed on the substrate of the element array,
An image forming apparatus, wherein the transistor circuit controls the luminance of the organic EL element based on a signal created by an area gradation method.   The drive circuit sets the luminance per unit time of the device EL element corresponding to the gradation control pixel to a value other than the maximum luminance or the minimum luminance among the unit cells of the electrostatic latent image formed by the exposure unit. 2. The image forming apparatus according to claim 1, wherein the luminance per unit time of the organic EL element corresponding to a pixel that is not the gradation control pixel is set to a maximum luminance value or a minimum luminance value.   The image forming apparatus according to claim 1, wherein the transistor circuit controls a value of a current flowing through the organic EL element.   The lens group is an inverted imaging system in a cross section perpendicular to the first direction, and is perpendicular to a second direction perpendicular to the first direction and the optical axis direction of each lens. 4. The image forming apparatus according to claim 1, wherein the image forming apparatus is an erecting equal-magnification imaging system in a cross section. The lens group includes a plurality of lens rows arranged in a second direction perpendicular to the first direction and the optical axis direction of each lens;
2. The optical axis of each of a plurality of lenses constituting the plurality of lens rows is spaced apart in the first direction and located on the same line between adjacent lenses. 5. The image forming apparatus according to any one of 4 above.   The image forming apparatus according to claim 1, wherein the lens group includes a first optical system and a second optical system that are separated from each other in the optical axis direction of each lens.   The image forming apparatus according to claim 6, wherein the first optical system and the second optical system have a symmetrical shape with respect to an intermediate imaging plane of the lens. The lens group passes through the first optical system and enters the second optical system in a cross section perpendicular to the second direction perpendicular to the first direction and the optical axis direction of each lens. The image forming apparatus according to claim 6, further comprising a light shielding member that shields part of the luminous flux to be shielded.